Tar sands, which are also known as oil and bituminous sands, are siliceous materials which are primarily silica (e.g., sand grains) having closely associated therewith an oil film. Tar sands generally comprise from about 5 to 21 percent by weight of an oil film; from about 70 to about 90 percent by weight of mineral solids; and from about 1 to about 10 percent by weight of an aqueous phase (i.e., water). The oil is very viscous, having an API gravity of about 1 to about 10 degrees API gravity, and typically comprises from about 1.0 to about 10.0 percent by weight inorganic elements (e.g., sulfur) and from about 30.0 to about 50.0 percent by weight cyclic hydrocarbons such as aromatics. The term "solids" is used herein to describe material of inorganic origin such as sand, clay and the like, as distinguished from materials of organic origin such as coke. The major portion, by weight, of the mineral solids in tar sands is quartz sand having a particle size greater than about 40 microns and less than about 2000 microns. The remaining mineral solid material found in tar sands has a particle size of less than about 40 microns and has been generally referred to as "fines". Fines comprise clay and silt including some very small particles of sand. Clay is a hydrated aluminum silicate with a generalized formula Al.sub.2 O.sub.3 SiO.sub.2.xH.sub.2 O. More specifically, it has properties of fine, irregularly shaped crystals and depending on the iron oxide content, it has a specific gravity of from about 2.3 to about 2.7. Types of clay are kaolinite, montmorrillonite, illite, bentonite, attapulgite, and halloysite. The fines content will vary from about 5 percent to about 35 percent by weight of the total mineral solid content of tar sands. It is to be understood and is not uncommon for the ingredients of tar sands to vary from the stated proportions and concentrations.
Canada has the potential to be self-sufficient in petroleum and could be in a very enviable position. To attain self sufficiency with her energy requirements increasing and conventional petroleum reserves decreasing annually, development of energy sources, such as the oil or tar sands, becomes essential. Major impediments to current development of conventional oil sands plants are the large capital investment requirement estimated to range from $35,000 to $45,000 per daily barrel of plant capacity and the concomitant operating costs associated with mining, extraction and upgrading of bitumen to saleable synthetic crude or transportation fuels. Equally important in the production of synthetic crude oil from oil sand by conventional first generation extraction technology is the large quantity of waste water and sludge produced which are likely to be unacceptable in future plants. For example, in the first 10 years of operation at a well known facility, the volume of waste water containment which stores process affected water, waste, process chemicals and sludge grew to about 215.times.10.sup.6 m.sup.3 and covers an area of 22 km.sup.2. (see "Development of the Tailings Pond at Syncrude's Oil Sands Plant; 1978-1987, " M. D. MacKinnon, AOSTRA Journal of Research, 5 (1989) pg. 109.) Future liabilities related to environmental issues and specifically waste water containment may also adversely affect future participation by industry.
The largest and most important deposits of tar sands are the Athabasca tar sands, found along the banks of the Athabasca River in the Province of Alberta, Canada. Total recoverable reserves from the Athabasca tar sands, after extraction and processing, have been estimated at more than about 300 billion barrels. Athabasca tar sands comprise sand grains which are each sheathed in a thin layer of aqueous phase (i.e. water). Bitumen is trapped in the void space between the water-wet grains. The composition of Athabasca tar sands may typically comprise from about 5 to about 15 percent by weight bitumen, from about 1.0 to about 10.0 percent by weight water, and from about 80.0 to about 90.0 percent by weight of solids (as defined above). While the present invention will be described with reference to Athabasca tar sands as a feed stock, it is to be understood that the spirit and scope of the present invention includes any hydrocarbon bearing sand as feed stock.
Presently known commercial arrangements include the existing Syncrude and Suncor operations in Alberta, Canada. Both operations employ dry mining of tar sands, which broadly comprises transportation of this material to extraction plants located distally, separation of a large sand fraction from the tar sands, and slurry transportation of the reject sand-clay-silt stream to a distal tailings pond. The Syncrude and Suncor operations broadly involve mining and tar sands transportation, followed by primary separation which incorporates tumblers, thickeners and flotation vessels. The tailings are slurried back to an ever-increasing pond due to the physical and chemical characteristics of the sands and sludge.
Athabasca tar sands are presently and more specifically processed using a method which is commonly referred to as the "hot water" method. In accordance with the "hot water" process for primary extraction of bitumen from tar sands, tar sand is mixed in a conditioning drum or tumbler with hot water and steam. Sodium hydroxide or other reagents are added as required to maintain and control the pH in the range of from about 8.0 to about 8.5. While in the tumbler, the tar sands disintegrates and liberates bitumen, and simultaneously, liberated bitumen is aerated. By "disintegrate" is meant that the bitumen particles and particulate sand are dispersed or separated from each other in a preliminary manner or fashion. Stated alternatively, during slurrying bitumen films are ruptured and a preliminary separation of the sand grains and bitumen droplets takes place. Simultaneously, air bubbles are entrained in the slurry. The tumbler produces a pulp or slurry comprising from about 5.0 to about 15.0 percent by weight bitumen, from about 15.0 to about 30.0 percent by weight water, and from about 60.0 to about 80.0 percent solids (as defined above), at a temperature of 180.degree.-195.degree. F.
The produced conditioned pulp or slurry leaves the tumbler and is then diluted with additional hot water to further disperse the sand and bitumen. This dilution operation is usually carried out at a screen positioned at the outlet from the tumbler. The diluted or flooded slurry typically has a composition comprising 2 to 10 wt. percent bitumen, 40 to 50 wt. percent water, and 45 to 55 wt. percent solids ( as defined above).
The diluted or flooded slurry is subsequently introduced into a separator cell in proximity to its longitudinal point. The separator cell is typically a cylindrical vessel having a conical bottom. The contents of the separator cell are commonly maintained at about 75.degree. to 85.degree. C. In the separator cell the bitumen particles, which have been attached to air bubbles, tend to rise to the surface of the water body and form a coherent mass known as an oily primary froth which for further treatment is recovered in a launder running around the rim of the cell. The major portion of the solids, particularly the coarse sand particles, tend to sink to the bottom of the cell and are withdrawn or drawn off as tailings.
A middlings stream comprising water, fine solids (e.g., clay) and some oil or bitumen, is continuously withdrawn from the separator cell at a point intermediate to the upper and lower ends of the separator cell. This middling stream typically comprises from about 1 to about 4 percent by weight bitumen, from about 10 to about 20 percent by weight mineral solids, and from about 75 to about 90 percent by weigjht aqueous phase or water. This middling stream is treated in a sub-aerated flotation cell to recover the contained bitumen in the form of secondary froth. The secondary froth is badly contaminated with mineral solids and water and may typically comprise 12 to 20 percent by weight solids, 50 to 60 percent by weight aqueous phase (or water) and 25 to 37 percent by weight bitumen. Once the bitumen has been extracted from the tar sand in this fashion, the primary and secondary froths are combined, diluted with a specific gravity-altering diluent (such as naphtha or any other suitable agent) and treated in a centrifuge circuit to separate the solids or residual minerals and water from the bitumen. The specific gravity-altering diluent (i.e., naphtha) may be distilled for further processing. The following patents broadly teach or suggest an apparatus and/or method for removing bitumen from tar sands, with some of the following patents more specifically teaching the caustic "hot water" process: U.S. Pat. No. 3,068,167 to White; U.S. Pat. No. 3,297,568 to McMahon; U.S. Pat. No. 3,392,105 to Poettmann et al; U.S. Pat. No. 3,696,923 to Miller; U.S. Pat. No. 3,864,251 to Cymbalisty; U.S. Pat. No. 3,869,384 to Schutte; U.S. Pat. No. 4,018,664 to Bain et al; U.S. Pat. No. 4,229,281 to Alquist et al; U.S. Pat. No. 4,340,487 to Lynn; U.S. Pat. No. 4,368,111 to Siefkin et al; U.S. Pat. No. 4,392,941 to Roth et al; U.S. Pat. No. 4,514,305 to Filby; U.S. Pat. No. 4,891,131 to Sadeghi et al; U.S. Pat. No. 4,913,805 to Chin; U.S. Pat. No. 4,946,597 to Sury; U.S. Pat. No. 4,968,412 to Guymon; U.S. Pat. No. 5,009,773 to Schramm et al; U.S. Pat. No. 5,019,245 to Ignasiak et al; U.S. Pat. No. 4,036,732 to Irani et al; U.S. Pat. No. 4,071,434 to Gifford; U.S. Pat. No. 4,110,194 to Peterson et al; U.S. Pat. No. 4,347,118 to Funk et al; U.S. Pat. No. 4,383,914 to Kizior; U.S. Pat. No. 4,399,039 to Yong; U.S. Pat. No. 4,424,112 to Rendall; U.S. Pat. No. 4,676,889 to Hsieh et al; U.S. Pat. No. 4,702,487 to Stoian et al; U.S. Pat. No. 4,719,008 to Sparks et al; U.S. Pat. No. 4,776,949 to Leung et al; and U.S. Pat. No. 4,929,341 to Thirumalachar et al. All of these U.S. Patents will be fully incorporated herein by reference thereto as if fully repeated verbatim immediately hereafter.
Conventional extraction processes result in a significant quantity of organic and inorganic sludges remaining with the reject tailings. This strongly bound (toluene insoluble) organic matter modifies the hydrophilic character of certain oil sand solids resulting in serious problems in processing oil or tar sands in conventional hot water extraction processes. ("Properties of Fines Size Fractions in Relation to the Distribution of Humic-Inorganic Matter Complexes in Athabasca Oil Sands," L. S. Kotlyar and B. D. Sparks, AOSTRA Journal of Research, 4 (1988) pg. 277.) These compounds also complicate the operation of the tailings pond by remaining in suspension as "globules." It has been reported that one half of the silt and clay and almost all the bitumen remains in suspension and flows to the center of the pond to form a sludge containing up to 85 to 90% process water.
Speight and Moschopedis presented results of studies addressing clays remaining in pond water in the form of a colloidal dispersion which adversely affects the volume of water available from the pond for recycle. Variation of pH causes a change in the charge on the surfaces of clay particles, thus effecting flocculation. (see "Factors Affecting Bitumen Recovery by the Hot Water Process, " J. G. Speight and S. E. Moschopedis, Alberta Research Council, 1978.) They further conclude that the disposal of tailings from the hot water extraction process represents one of the major problems facing commercial development. While surface active materials present in the bitumen appear to have a beneficial effect on bitumen recovery, their ability to act as clay dispersants has an adverse effect on the settling of the clays in the tailings pond. (see "Surface and Interfacial Phenomena Related to the Hot Water Processing of Athabasca Oil Sands," J. G. Special and S. E. Moschopedis, Alberta Research Council, Information Series 86, 1980.)
Bowman and Co-workers (J. Leja and C. W. Bowman, "Application of Thermodynamics to the Athabasca Tar Sands", Can. J. Chem. Eng., A6 (1968) pg. 479) established that the surface active agents in the process are primarily water soluble salts of naphthionic acids having carboxylic functional groups. Furthermore, they observed that the surfactants interact with mineral surfaces and play a role in solids flotation.
Sanford and Co-workers concluded that the role of sodium hydroxide in the hot water extraction process is primarily that of a generator of natural surfactants which in some way aid oil separation and/or flotation. A further finding and of equal importance was the relation between the fines level and the caustic needed for oil recovery. Furthermore, very lean grades of oil sands may not be able to supply enough surfactant, adversely affecting bitumen recovery. (L. L. Schramm, R. G. Smith, J. A. Stone AOSTRA Journal of Research, Vol. 1, 1984; page 10)
Schramm and Smith conducted extensive testing to determine the adverse affects that aging of tar sands had on bitumen recovery. They concluded that the aging mechanism is traced to changes in natural surfactant concentrations generated during processing. In essence they found that the aging effect can be traced to reactions that effectively reduce the concentration of natural carboxylate surfactant produced in the hot water separation process. ("Some Observations on the Aging Phenomenon in the Hot Water Processing of Athabasca Oil Sands, Part 2, The Mechanism of Aging," L. L. Schramm and R. G. Smith, AOSTRA Journal of Research, Vol. 3 (1987) pg. 215.)
Kotlyar, Sparks and Kodama concluded that serious problems which occur during bitumen extraction by the hot water process could be due to the fact that the hydrophilic (water loving) character of some of the solids is modified by the presence of tightly bound organic (humic) matter. This material cannot be removed by extraction with good solvents commonly used for bitumen such as toluene or dichloromethane. Most of the humic matter in oil sands is known to be associated with fines i.e., that fraction of oil sands solids with a particle size below 38 microns. (see "Isolation and Characterization of Organic-Rich Solids Present in Athabasca Tailings Pond Sludge", L. S. Kotlyar, B. D. Sparks and H. Kodaman, AOSTRA Journal of Research, Vol. 6 (1990) pg. 41.)
M. D. MacKinnon addresses the development of the tailings pond at the Syncrude plant between 1978 and 1987. He indicates that about 70% of the plant water requirements are reclaimed from the free water zone and that 1 m.sup.3 of water per ton of oil sand is recycled from the pond. An additional requirement of 0.3 m.sup.3 per ton of oil sand is withdrawn from the Athabasca River. Extensive information on the physical and chemical properties of the pond is presented. (M. D. MacKinnon, AOSTRA Journal of Research, Vol. 5 (1989) pg. 109-131.) Other organizations have also investigated alternative technologies for the surface minable resource with a view to reducing the cost of recovering bitumen from oil sands as well as minimizing some of the problems noted.
As specifically stated in U.S. Pat. No. 4,392,941 to Roth et al, the tailings that are collected from recovering bitumen from tar sands, generally will contain solids as well as dissolved chemicals. The tailings are usually collected in a retention pond where additional separation occurs. As is well known, retention ponds involve large space requirements and the construction of expensive enclosure dikes. The tailings can also be considered as processing waste water containing solids which are discharged from the extraction process. The tailings comprise waste water, both the natural occurring water and added water, bitumen and mineral. As stated in U.S. Pat. No. 4,018,664 to Bain et al, because this waste water contains oil emulsions, finely dispersed clay with poor settling characteristics and other contaminants, water pollution considerations prohibit discarding the effluent into rivers, lakes or other natural bodies of water. The disposal of the waste water streams has therefore presented a problem. A portion of the water in the waste water stream can be recycled back into the hot water extraction process as an economic measure to conserve both heat and water. However, experience has shown that the dispersed silt and clay content of the recycled water can reduce primary froth yield by increasing the viscosity of the middlings layer and retarding the upward velocity of oil droplets. When this occurs, the smaller oil droplets, and those that are more heavily laden with mineral matter stay suspended in the water of the separation cell and are removed from the cell with the middlings layer.
The mineral particle size distribution is particularly significant to operation of the hot water process and to sludge accumulation. The terms "sand", "silt" and "clay" are used in this specification as particle size designations. Sand is siliceous material which will not pass through a 325 mesh screen. Silt will pass through a 325 mesh screen but is generally smaller than 45 microns and larger than two microns and can contain siliceous material. Clay is smaller than 2 microns and also can contain siliceous material. The word "fines" as used herein refers to a combination of silt and clay.
As previously indicated and as specifically stated in U.S. Pat. No. 4,392,941 to Roth et al, conditioning tar sands for the recovery of bitumen consists of heating the tar sand/water mixture to process temperature (180.degree.-200.degree. F.), physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) of the caustic (i.e., NaOH) or other added reagents. Under these conditions as Roth et al points out in U.S. Pat. No. 4,392,941, bitumen is stripped from the individual sand grains and mixed into pulp in the form of discrete droplets of a particle size on the same order as that of the sand grains. During conditioning, a large fraction of the clay particles becomes well dispersed and mixed throughout the pulp. The conditioning process which prepares bitumen for efficient recovery during the following process steps also prepares the clays to be the most difficult component to deal with in the tailings disposal operation.
As further previously indicated and as specifically stated by Roth et al in U.S. Pat. No. 4,392,941, the other process step, termed "separation", is actually the bitumen recovery step because the separation has already occurred during conditioning. The conditioned tar sand pulp is screened to remove rocks and unconditionable lumps of tar sands and clay. The reject material, termed "screen oversize", is discarded. The screened pulp is further diluted or flooded with water to promote the following two settling processes: globules of bitumen, essentially mineral-free, float upward to form a coherent mass of froth on the surface of the separation units; and, at the same time, mineral particles, particularly the sand size material, settle down and are removed from the bottom of the separation unit as sand tailings. These two settling processes take place through a medium called the middlings. The middlings consists primarily of water, bitumen particles and suspended fines which includes silt and clay.
Roth et al in U.S. Pat. No. 4,392,941 has stated that the particular sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter which influences the settling processes most is the viscosity of the middlings. Characteristically, as the suspended material content rises above a certain threshold, which varies according to the composition of the suspended fines, viscosity rapidly achieves high values with the effect that the settling processes essentially stop. Little or no bitumen is recovered and all streams exiting the unit have about the same composition as the feed. As the feed suspended fines content increases, more water must be used in the process to maintain middlings viscosity within the operable range.
The third step of the hot water process is scavenging. The feed suspended fine content sets the process water requirement through the need to control middlings viscosity which, as noted before and as indicated by Roth et al in U.S. Pat. No. 4,392,941, is governed by the clay/water ratio. It is usually necessary to withdraw a stream of middlings to maintain the separation unit material balance, and this stream of middlings can be scavenged for recovery of incremental amounts of bitumen. Air flotation is an effective scavenging method for this middlings stream.
As is well known in the art, final extraction or froth clean-up is usually accomplished by centrifugation. Froth from primary extraction is diluted with naphtha, and the diluted froth is then subjected to a two stage centrifugation. This process yields an oil product of essentially pure, but diluted, bitumen. Water and mineral and any unrecovered bitumen removed from the froth constitutes an additional tailing stream which must be disposed.
Tailings are a throwaway material generated or obtained in the course of extracting the valuable material (i.e. bitumen) from the non-valuable material (e.g. sand, sludge, etc.) And in tar sands processing, tailings consist of the whole tar sand plus net additions of process water less only the recovered bitumen product. Roth et al in U.S. Pat. No. 4,392,941 has subdivided tar sand tailings into the following three categories: (1) screen oversize; (2) sand tailings--the fraction that settles rapidly, and (3) middlings--the fraction that settles slowly. Screen oversize is typically collected and handled as a separate stream.
Tailings disposal is the operation required to place the tailings in a final resting place. As previously indicated, because the tailings contain bitumen or oil emulsions which may be defined as finely dispersed clay with poor settling characteristics and other contaminants, water pollution considerations prohibit discarding the tailings into rivers, lakes or other natural bodies. As previously mentioned, currently the tailings are stored in retention ponds (also referred to as evaporation ponds) which involve large space requirements and the construction of expensive enclosure dikes. As further previously mentioned, a portion of the water in the tailings can be recycled back into the water extraction process as an economic measure to conserve water. Roth et al in U.S. Pat. No. 4,392,941 has indicated that the following are two main operating modes for tailings disposal: (1) dike building--hydraulic conveying of tailings followed by mechanical compaction of the sand tailings fraction; and (2) overboarding--hydraulic transport with no mechanical compaction.
For dike building at a well known commercial location, tailings are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impoundment for a pool of liquid contained inside. On the dike, sand settles rapidly and a slurry of water, silt, clay and minor amount of bitumen, as well as any chemical used during processing flows into the pond interior. The settled sand is mechanically compacted to build the dike to a higher level. The slurry which drains into the pond interior commences stratification in settling over a time scale of months to years. As a result of this long term settling, Roth et al in U.S. Pat. No. 4,392,941 has stated that three layers form. The top layer, e.g. 5-10 feet of the pool, is a layer of relatively clear water containing minor amounts of solid, e.g. up to 5 wt. percent and any dissolved chemicals. This layer of pond water can be recycled to the water extraction process without interfering with extraction of bitumen from tar sands. As previously indicated, recycling pond water serves to reduce the overall volume increase of water stored in retention pond. Below this clear top water layer is a discontinuity in solid contents. Over a few feet, solids content increases to about 10-15 wt. percent and thereafter, solids contents increase regularly toward the pond bottom. In the deeper parts of the pond, solid contents of over 50 wt. percent have been measured. This second layer is commonly called the sludge layer. In general the sludge layer can be characterized as having more than 10 wt. percent solids (which may also be defined as mineral plus bitumen). More particularly as Roth et al has stated in U.S. Pat. No. 4,392,941, the sludge can be characterized as having 20 wt. percent to 50 wt. percent solids or mineral matter comprising substantially clay and silt. Also the sludge can be characterized as having about 0.5 to about 25 wt. percent bitumen. The solids contents of the sludge layer increase regularly from top to bottom by a factor of about 4-5. Portions of the solids are clays. The clays, dispersed by the processing, apparently have partially reflocculated into a fragile gel network. Through this gel, particles of larger-than-clay sizes are slowly settling. Generally this sludge layer cannot be recycled to the separation step because no additional bitumen is extracted. More specifically, sludge is not suitable for recycling to the hot water extraction process for the reason that its addition into the separation cell or the scavenger cell at the normal inlet means would raise the mineral content of the middlings of the cell to the extent that recovery of bitumen would be substantially reduced. Generally, the settling which does take place in the pond provides a body of water in which the concentration of mineral matter increases substantially from the surface of the pond to the bottom thereof. A third layer formed of sand also exists.
Roth et al has defined in U.S. Pat. No. 4,392,941 "overboarding" as the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. A rapid and slow settling process occurs but this distinction is not as acute as in the previously mentioned dike building and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to form a gently sloping beach, extending from the discharge point toward the pond interior. As the sand settles, a slurry drains into the pool and commences long-term settling. Thus water in ponds prepared by both dike building and overboarding can be included in the general definition of sludge in the present description.
As stated in U.S. Pat. No. 4,018,664 to Bain et al, experience has shown that, as the pond forms, the various components in the effluent discharge settle in the pond at varying rates. As an example, when the waste water containing sand, silt, clay and bitumen is discharged to the pond, the free bitumen normally immediately floats to the surface of the pond and the sand immediately settles to the bottom of the pond. However, after the surface bitumen cools and releases the entrapped air which originally caused it to float, it too will begin to settle toward the bottom of the pond. The silt and clay in the discharge settle in the pond at a substantially low rate as compared to the sand.
Bain et al has characterized a pond in U.S. Pat. No. 4,018,664 as follows: it can be pictured as a large body of water containing dispersed solids which are slowly settling toward the bottom of the pond. The mineral matter in the pond is in a constant but slow state of settling. Normally, the pond is constantly increasing in size because of the continuous addition of waste water and therefore the character of the pond is continually changing.
In processing tar sands to recover bitumen therefrom, the tar sands are excavated, extracted to remove the bitumen, whereafter the sand and other minerals are returned to the excavated area. As noted above, waste waters associated with the extraction step must be stored in a retention pond which is normally placed in one of the excavated areas. It is important that the excavated area be filled only with minerals and not with water since obviously the water is excess and therefore requires more storage volume than is available. If a retention pond associated with the hot water extraction of bitumen from tar sands is not treated to remove water layers which cannot normally be reused, such as sludge, the problem of a shortage of storage space is ever present.
Bain et al has further characterized a pond in U.S. Pat. No. 4,018,664 as follows: a waste water retention pond associated with hot water process for extracting bitumen from 140,000 to 150,000 tons of tar sands per day and having a surface area of about 1,000 acres and an average depth of 40 feet can be characterized somewhat as follows:
(a) From the surface of the pond to a depth of about 15 feet the mineral concentration which is primarily clay is found to be about 0.5 to 5.0 weight percent. This pond water can normally be recycled to a hot water extraction process without interfering with the extraction of bitumen from tar sands.
(b) The layer of water in the pond between 15 and 25 feet from the surface contains between 5.0 and 20 percent mineral matter. This water, if recycled to the separation cell feed with fresh tar sands, would increase the mineral content of the middlings portion of the cell to the point that little bitumen would be recovered.
(c) Finally, the section of the pond between 25 feet and the bottom of the pond contains 20 to 50 percent mineral matter and is normally referred to as sludge.
What is needed and what has been discovered by us is a process for the recovery of hydrocarbon or bitumen from tar sands. More specifically, what is needed and what has been invented by us is a process for the recovery of hydrocarbon or bitumen from tar sands and the rejection of recovered sand from the tar sands, all at the mining site. The recovered hydrocarbon or bitumen may be upgraded by sending the same to an upgrading unit.